Look I know I am not a hockey expert and, like most here, I'm just a spectator but I do read and listen to what the actual hockey experts, analysts, and professionals have to say and the consensus is that Glass and Vilardi were higher ranked prospects than Pettersson.
In the case of Glass he is a very safe player to pick with a very high ceiling (Mark Scheifele and Patrice Bergeron). What that is your BPA in a position the organization needs to fillout mid and long term, why go for someone with a higher risk and basically "a project"